These words both relate to the internal constitution of bodies; but hardnotes a more impenetrable nature or a firmer adherence of the component parts than solid. Hard is opposed to soft, and solid to fluid, liquid, open, or hollow.

Webster's Revised Unabridged Dictionary (1913)

Example sentences

concrete Sami was sleeping on a concrete floor in the dark.
substantial Human vices provide substantial government income.
compact Male gorillas are compact and powerful creatures.
dense Not a soul was to be seen because of a dense fog.
nourishing This fish is inexpensive but nourishing.
unbroken The rain was unbroken for the whole of last week.

Popularity over time

Word	1850 → 2019	Trend
solid		steady
concrete		falling
substantial		falling
compact		falling
dense		steady
nourishing		rising

Use in printed books, per decade from 1850 to 2019.

By length

3 letters: bad¹
4 letters: just¹, good¹, real¹, hard¹, true¹, firm¹
5 letters: whole¹, close¹, sound¹, heavy², tough¹, thick¹, dense¹, rigid², cubic², stout¹, meaty²
6 letters: strong¹, entire³, closed¹, square¹, stable², steady², hearty²
7 letters: massive², compact²
8 letters: material⁴, concrete², reliable⁴, sterling², unbroken³
9 letters: unanimous⁴, undivided⁴
10 letters: continuous⁴, satisfying⁴, nourishing³, upstanding³
11 letters: substantial³, homogeneous⁵
13 letters: uninterrupted⁵

The small number after each word is its number of syllables (heavy² = 2 syllables).

In other languages

Français: solide
Italiano: solido
Español: sólido
Deutsch: solide